Continuous ink jet (also commonly referred to as continuous stream, etc.) printing systems, use a pressurized ink source and a drop forming mechanism for producing a continuous stream of ink drops. Conventional continuous ink jet printers utilize electrostatic charging devices that are placed close to the point where a filament of working fluid breaks into individual ink drops. The ink drops are electrically charged and then directed to an appropriate location by deflection electrodes having a large potential difference. For example, when no printing is desired, the ink drops (non-printed drops, etc) are deflected into an ink capturing mechanism (catcher, interceptor, gutter, etc.) and either recycled or discarded while non-deflected ink drops (printed drops, etc.) are permitted to contact a recording media. Alternatively, printed ink drops can be deflected toward the recording media while non-deflected non-printed ink drops travel toward the ink capturing mechanism.
As drops are continuously being formed and selectively deflected during operation, print quality and system performance in continuous ink jet printers is particularly sensitive to variations in drop volume (drop size, etc.). Variations in drop volume can cause the printed dot size on the recording media to vary which can adversely affect print quality. For example, when the volume of ejected drops increases or decreases while a page of recording media is being printed, the colors printed at the top of the page can be inconsistent with the colors printed at the bottom of the page. This can affect the darkness of black-and-white text, the contrast of gray-scale images, and the saturation, hue, and lightness of color images. Additionally, variations in drop volume can adversely affect system performance. For example, the drop deflection mechanism may not consistently deflect drops when the drop volume varies. This can result in an increase or a decrease in the deflection angle causing drops to be deflected too much or not enough.
A change in ink viscosity caused by, for example, a change in operating temperature can cause drop volumes to vary. While changes in ink viscosity caused by the evaporation of the solvent component of the ink composition can be compensated for measuring either the optical absorbency or the electrical conductivity of the ink and adding make-up solvent accordingly, ink viscosity is also a function of temperature. For example, a drop forming mechanism that provides drops having a desired volume at normal ambient room temperature (e.g., 60°-82° F.) can provide drops having a larger undesired volume when the surrounding temperature increases (e.g., 85°-95° F.). The extra ink provided by the drop forming mechanism degrades the print quality by causing an increase in the density of the printed dot. Alternatively, the drop forming mechanism can provide drops having a smaller undesired volume when the surrounding temperature decreases which can also degrade print quality.
Even when the printer is located in a room that is successfully maintained within a normal ambient temperature range, the temperature of the printhead housing the drop forming mechanism can increase beyond acceptable ambient temperatures due to, for example, the heat generated by forming and/or deflecting the drops. Again, this produces a variation in drop volume which can adversely affect print quality. In these situations, adding solvent or ink concentrate to the ink composition to compensate for the temperature induced viscosity changes produces an ink composition having unintended property changes, for example changes in optical density and, as such, is an inadequate solution to the problem.
U.S. Pat. No. 5,623,292 issued to Shrivastava et al. on Apr. 22, 1997, provides a temperatures control unit in a printhead in order to control ink temperature. The temperature control unit includes a heat pump assembly coupled to a heat exchanger through which the ink flows. However, this solution is disadvantaged in that it requires additional hardware for the heating and/or cooling the ink which increases the cost of the printer. Additional time is also required prior to printing in order to permit the ink to reach a desired temperature.
As such, there is a need to be able to monitor changes in ink parameters (for example, ink viscosity) caused by changes in operating conditions (for example, temperature) in order to compensate for inconsistencies in drop volumes without controlling the temperature of the print head.